As used herein, a fuel processing assembly is a device or combination of devices that produces hydrogen gas from one or more feed streams that include one or more feedstocks. Examples of fuel processing assemblies include steam and autothermal reformers, in which the feed stream contains water and a carbon-containing feedstock, such as an alcohol or a hydrocarbon. Fuel processors typically operate at elevated temperatures. In endothermic fuel processing reactions, such as in steam reforming fuel processing assemblies, the heat required to heat at least the hydrogen-producing region of the fuel processing assembly to, and maintain the region at, a suitable hydrogen-producing temperature needs to be provided by a heating assembly, such as a burner, electrical heater or the like. When burners are used to heat the fuel processor, the burners typically utilize a combustible fuel stream, such as a combustible gas or a combustible liquid.
In a hydrogen-producing fuel processing assembly that utilizes a steam reformer, or steam reforming region, hydrogen gas is produced from a feed stream that includes a carbon-containing feedstock and water. Steam reforming is performed at elevated temperatures and pressures, and a steam reformer typically includes a heating assembly that provides heat for the steam reforming reaction. Illustrative but not exclusive uses of the heat include maintaining the reforming catalyst bed at a selected reforming temperature, or temperature range, and vaporizing a liquid feed stream prior to its use to produce hydrogen gas. One type of heating assembly is a burner, in which a combustible fuel stream is combusted with air. In a hydrogen-producing fuel processing assembly that utilizes an autothermal reformer, or autothermal reforming region, hydrogen gas is produced from a feed stream that includes a carbon-containing feedstock and water, which is reacted in the presence of air. Steam and autothermal reformers utilize reforming catalysts that are adapted to produce hydrogen gas from the above-discussed feed streams when the hydrogen-producing region is at a suitable hydrogen-producing temperature, or within a suitable hydrogen-producing temperature range. The product hydrogen stream from the hydrogen-producing region may be purified, if needed, and thereafter used as a fuel stream for a fuel cell stack, which produces an electric current from the product hydrogen stream and an oxidant, such as air. This electric current, or power output, from the fuel cell stack may be utilized to satisfy the energy demands of an energy-consuming device.
A consideration with any hydrogen-producing fuel cell system is the time it takes to begin generating an electric current from hydrogen gas produced by the fuel cell system after there is a need to begin doing so. In some applications, it may be acceptable to have a period of time in which there is a demand, or desire, to have the fuel cell system produce a power output to satisfy an applied load, but in which the system is not able to produce the power output. In other applications, it is not acceptable to have a period where the applied load from an energy-consuming device cannot be satisfied by the fuel cell system even though there is a desire to have this load satisfied by the system. As an illustrative example, some fuel cell systems are utilized to provide backup, or supplemental power, to an electrical grid or other primary power source. When the primary power source is not able to satisfy the applied load thereto, it is often desirable for the backup fuel cell system to be able to provide essentially instantaneous power so that the supply of power to the energy-consuming devices is not interrupted, or not noticeably interrupted.
Fuel cells typically can begin generating an electric current within a very short amount of time after hydrogen gas or another suitable fuel and an oxidant, such as air, is delivered thereto. For example, a fuel cell stack may be adapted to produce an electric current within less than a second after the flows of hydrogen gas and air are delivered to the fuel cells in the fuel cell stack. Inclusive of the time required to initiate the delivery of these streams from a source containing the hydrogen gas and air, the time required to produce the electric current should still be relatively short, such as less than a minute. However, hydrogen-producing fuel cell systems that require the hydrogen gas to first be produced, and perhaps purified, prior to being utilized to generate the desired power output take longer to generate this power output. When the fuel processing assembly is already at a suitable hydrogen-producing temperature, the fuel cell system may be able to produce the desired power output from hydrogen gas generated by the fuel processing assembly within a few minutes, or less. However, when the hydrogen-producing fuel processor of the fuel cell system's fuel processing assembly is not already at a desired hydrogen-producing temperature, the required time will be much longer. For example, when started up from an ambient temperature of 25° C., it may take thirty minutes or more to properly start up the fuel processing assembly and to produce the desired power output from hydrogen gas produced by the fuel processing assembly.
Conventionally, several different approaches have been taken to provide hydrogen-producing fuel cell systems that can satisfy an applied load while the associated hydrogen-producing fuel processing assembly is started up from its off, or unheated and inactive, operating state, heated to a suitable hydrogen-producing temperature, and thereafter utilized to produce and optionally purify the required hydrogen gas to produce a power output to satisfy the applied load. One approach is to include one or more batteries or other suitable energy storage devices that may be used to satisfy the applied load until the fuel cell system can produce a sufficient power output to satisfy the applied load. Typically, this approach also requires that the fuel cell system include suitable chargers to recharge the batteries during operation of the fuel cell system. This approach is effective, especially for lower power demands of 1 kW or less, so long as the weight and size requirements of the battery, or batteries, is acceptable. In portable fuel cell systems and fuel cell systems that are designed to satisfy greater applied loads, such as loads of 10 kW or more, it may not be practical to utilize batteries to satisfy an applied load for the time required for the fuel processing assembly to be started up. Another approach is for the fuel processing assembly to include a hydrogen storage device that is sized and otherwise configured to store a sufficient amount of hydrogen gas to supply the fuel cell stack while the fuel processing assembly is started up. Typically, this approach also requires that the fuel cell system include suitable compressors and other control and regulation structure to recharge the storage device. This approach is also effective, but requires that the space, additional equipment and expense of including the storage device and associated components is acceptable.
In some applications, it may be desirable to be able to produce a desired power output from hydrogen gas produced by the fuel processing assembly of a hydrogen-producing fuel cell system without requiring either stored hydrogen or stored power to be used to satisfy the applied load while the fuel processing assembly is started up from an inactive, or off, operating state and heated to a suitable hydrogen-producing temperature.